4. For each of the following use a Karnaugh map to produce the minimal sum of products. For each shade each distinguished 1-cell. Also, if more than one possible minimal

answer is possible, give the possible answers. \text { a. } \quad F a=\sum(0,1,4,13,15)+u(3,5,7,14) \text { b. } \quad F b=\sum(3,5,10,12,14,15)+u(6,7,8,11) \text { c. } \quad F c=\sum(0,1,9,12,13,14)+u(5,8,15)